Colloidal self-assembly concepts for light management in photovoltaics
Matthias Karg
2, Tobias A.F. Ko¨nig
1, Markus Retsch
2, Christian Stelling
2,
Paul M. Reichstein
3, Tobias Honold
2, Mukundan Thelakkat
3and Andreas Fery
1,*
1PhysicalChemistryII,UniversityofBayreuth,Universita¨tsstr.30,95447Bayreuth,Germany
2PhysicalChemistryI,UniversityofBayreuth,Universita¨tsstr.30,95447Bayreuth,Germany
3AppliedFunctionalPolymers,MacromolecularChemistryI,UniversityofBayreuth,Universita¨tsstr.30,95447Bayreuth,Germany
Colloidal particles show interaction with electromagnetic radiation at optical frequencies. At the same time clever colloid design and functionalization concepts allow for versatile particle assembly providing monolayers of macroscopic dimensions. This has led to a significant interest in assembled colloidal structures for light harvesting in photovoltaic devices. In particular thin-film solar cells suffer from weak absorption of incoming photons. Consequently light management using assembled colloidal structures becomes vital for enhancing the efficiency of a given device. This review aims at giving an overview of recent developments in colloid synthesis, functionalization and assembly with a focus on light management structures in photovoltaics. We distinguish between optical effects related to the single particle properties as well as collective optical effects, which originate from the assembled structures.
Colloidal templating approaches open yet another dimension for controlling the interaction with light.
We focus in this respect on structured electrodes that have received much attention due to their dual functionality as light harvesting systems and conductive electrodes and highlight the impact of inter- particle spacing for templating.
Introduction
The world’s increasing energy demands along with gradually depletingfossil fuel stockand the detrimentaleffectsofglobal warmingrequirehighlyefficient energyconversionand storage fromrenewableenergysources.Photovoltaics(PV)isoneofthe maincontendersinthis field[1].PV offersabroad diversityof systemsrangingfrom thefirst generationopaquesinglecrystal siliconsolarcellsuptothirdgenerationsemitransparentorganic and hybrid systems. The former absorbalmost all the incident lightenergywithinthe bandgapofthematerialandconvertit efficientlyintoelectricalenergyunderhighlightintensitycondi- tionsand thereforearemainlyapplied onroof topsor in solar energy parks. Here the main losses arise from reflection and thermalrelaxation of hotelectrons. Inthe latter, only a small fractionofincidentlightisabsorbedandthesesystemsworkalso
verywellunderdiffuselightconditionsmakingthemsuitablefor indooraswellasbuildingintegratedapplications[2].Additionally, theselightweightthinfilmsolarcellscanbefabricatedonflexible rollsusingprintingtechnologiesandaresuitableforlamination oncurvedsurfacesorforautomotiverooftops.However,itisa challenge to fabricate semitransparent solar cells maintaining high power conversion efficiency. This is due to the fact that the loss channels in thin film organic and hybrid systemsare multifaceted; the transmissionand reflection losses covering a majorpartofit.Thethicknessofthelightharvestingandcharge transportinglayersinsuchsystemsarelimitedto<200nmdueto therestrictionsofsmallexcitondiffusionlengthandlowcharge carrier mobility. Thus,a maindrawback of suchdevices is the comparablythinactivelayerthatleadstoratherweakabsorption characteristics.Consequentlythelosschannelswithintheoptical gaphavetobeminimizedinordertoimprovethepowerconver- sionefficiency. Herelightmanagementusinga combinationof
RESEARCH:Review
*Correspondingauthor:Fery,A.(andreas.fery@uni-bayreuth.de)
1369-7021/ß2014TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/3.0/).http://dx.doi.org/10.1016/
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differentapproachesisanecessitytoimprovetheperformanceof thedevices.
Much effort in recent years was devoted to the design and developmentofnewactivematerialstoimprovetheoveralllight harvesting[3]byextendingtheopticalgaptotheredregionofthe solar spectrum. Energy transfer within the absorbing layer via Fo¨rsterresonanceenergytransfer(FRET)[4,5]andco-sensitization in dye-sensitizedsystemswerealsodemonstrated[6,7].Inaddi- tion,independentofthedevicetype,generallightmanagement approachessuchasmicrocavity/opticalspacerforwaveguiding [8],useofscatteringlayers/structures[9]andplasmonicsarealso intensivelyinvestigated[10].
ThisreviewfocusesontheuseofcolloidalparticlesinPVforan efficientlightmanagement.Specificallyweaddresstwoaspectsin detail,section‘Colloidalstructuresforlightharvesting’:theuseof colloidal structures for light harvesting and section ‘Template electrodes from colloidalassembly structures’:their application astemplatesfortheassemblyofelectrodes,whichatthesametime havelightharvestingfunctionality.Weaimtogiveanoverviewof conceptsandrationalmaterialsdesignforusingcolloidalparticles toenhancetheperformanceofafullyfunctionalPVdevice,thatis, the colloids arenotsourceof the chargesgenerated. Thus,our discussiondoesnottakeintoaccounteffectssuchasdirectelec- tronextraction from,forexample,metal nanoparticlessuch as goldandsilver[11–14]aswellasexcitonformationanddissocia- tionusingsemiconductorquantumdots[15–19].
Thefirstsectionstartswithanoverviewofthetypeofcolloidal structuresthathavesuccessfullybeenappliedforlightharvesting andcategorizesthemintermsoftheiruseinspecificPVsystems andstructureonthecolloidalscale,thussettingthesceneforthe detaileddiscussion inthefollowingsections.Colloidal particles havedimensionsontheorderorsmallerthanthewavelengthof visible light and hence they possess size, shape and material dependent opticalproperties.Section‘Role ofparticlesinPV’is dedicatedtoanin-depthdiscussionoftheseopticalproperties.In contrasttoconventionallithographicapproaches,whichareusu- allyusedtocreatenano-andmesoscalelightmanagementstruc- tures,colloidal particlesaretypicallyprepared viawet-chemical synthesis.Duringthelastcentury,aplethoraofprotocolsforsize- andshape-selectivesynthesisofcolloidalparticlesfromdifferent materials has been developed [10],[20–23]. In section ‘Particle design:size,shape,material’,wewillreviewthesynthesisofwell- defined colloidalparticles withspecialattention tothe require- mentsforlightharvestingapplications.Section‘Particlefunctio- nalization’dealswiththefunctionalizationofcolloidalparticles thatallowsforthecontrollingof(i)inter-particleinteractions,(ii) inter-particlespacingfromnanometersuptomicrons,(iii)com- plex colloidal surface assemblies and (iv) surface coverage. In section ‘Local and collective coupling effects’, the impact of assemblyonopticalpropertiesandtheappearanceofnoveloptical featuressuchaslocalinter-particlecouplingeffectsandcollective longrangeresonanceswillbediscussedindetail.Part2isdedicat- ed to ordered nano- and mesostructures obtained via colloidal templating.Herethecolloidalparticlesthemselvesdonotactas functional features, but rather colloidal assemblies are used as sacrificial masks for lithographic processes. This opens a new parameter space,where the functionality ofthe PV devicecan be custom designed starting with the supporting substrate or
electrodeinterface.Colloidallithographycanbecombined with sol–gel,evaporation,andreactive-ionetchingtechniques,which inmanycaseswillnotonlyaddhighlyorderednanostructurestoa PV system,but alsooften results ina defined topography with increasedsurfacearea.Section‘Structuredelectrodesfromclose- packed colloidal monolayers’illustrates the formation of struc- tured electrodes from close-packed colloidal monolayers. Here predominantlyindividuallyseparatednanostructuresareaccessi- ble.Insection‘Structuredelectrodesfromnon-close-packedcol- loidalmonolayers’,non-close-packedcolloidalmonolayerswillbe discussed, which can be used to obtain percolated structures, which forinstance allow the excitation of propagatingsurface plasmons,ormaybeusedasfuturetransparentconductingelec- trodematerials.
Colloidal structures for light harvesting
TheimplementationofcolloidsinPVdevicesrepresentsaprom- isingrouteforlightmanagementtasksinparticularforthin-film solarcells[10,20,21,24–28].Theopticaleffects,whichareachiev- ablebycolloidsaremanifoldandthey canbegroupedintothe followingtwotypesdependingontheparametersdefinedbythe natureofthebuildingblocksandtheassemblystructure.
(1) Parametersadjustablethroughthecolloiddesign:Composi- tion, size, shape, size- and shape-distribution, stability, functionalization,particlespacing.
(2) Parametersadjustablethroughtheassemblyprocess:Colloid distribution,surfacecoverage,inter-particledistance,period- icity.
Furthermorethelocationofthe particleswithinthedeviceis crucial for the optical performance and needs to be chosen dependingonthedesiredeffect[29].Figure1(a)illustratessche- matically the resulting refractive index conditions for particles being embedded either in the active medium (n1=n2) or for particles located at an interface suchas the interface between theelectrodeandtheactivematrix(n16¼n2).Forthesakeofclarity thesketchisoversimplified:Morethantwolayerswithdifferent refractiveindicesmightactuallyinfluenceopticalpropertiesofthe colloid.Furthermorethesurroundingmaterialsmaybedifferent in chemical composition but exhibit a nearly equal refractive index(n1n2) or provide an effectiveaverage refractive index environment.Inadditionthepositionofthecolloidatornearthe interface may differ from a contact angle of 908 as shown in Figure1(a).Table1givesanoverviewofexemplarystudieswhere thedifferentrefractiveindexconditionsareutilizedindevicesfor thescenariosofn1=n2,n1n2,andinterfacen16¼n2.
Anotherimportantfactor,whichdeterminestheopticalbehav- iorofcolloidsinPVdevicesistheparticledistribution.Figure1(b) showsschematicallyrandomdistributionsofsingleorclustered particles.Inarandomarrangementofsingleparticlesanaverage nearestdistancecanbedetermined.Ifthesedistanceslismuch largerthantheparticlediametertheopticalpropertiesofthelayer willresemblethepropertiesofthesinglebuildingblocks.Thus,no cooperativeeffectsarefound.Thissituationisdifferentforran- domlydistributedclustersconsistingoftwoormoreparticleswith acenter-to-centerdistanced.Atsmallvaluesofdtheclusterswill significantlydifferin their opticalresponse comparedto single particles.Thedetailedeffectsofparticlesincloseproximitywillbe discussedinsection‘Localandcollectivecouplingeffects’.
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Besidesrandomarrangementsofparticlesasanopticallyfunc- tional layer, introducing order can lead to additional effects.
Figure 1(c) shows examples for 1D periodic structures (particle lines)andnon-close-packedhexagonal2Dmonolayers.Inthe1D exampletwolengthscalesbecomeimportantbecausetheparticle patternhasnorotationalsymmetry:Thecenter-to-centerdistance dofparticleswithintheparticlechainandtheperiodicityl,which isthe line-to-line separationinthis example.For l=da square structureresults.The2Dhexagonalmonolayershowsrotational symmetryandisconsequentlycharacterizedbyonelengthscalel, which is the lattice constant. If lequals the particle diameter, neighboringparticlesareincontactandaclose-packedmonolayer is obtained. The given examples shall illustrate the variety of structuresandrelevantlengthscales.
Theoretical studies can helpto decidefor the mostpromising design rules. Qu et al. investigated the influence of the particle
positiononthesolarcellperformance[46].Themostefficientloca- tionturnedouttobetheinterfacebetweenapoly-3,4-ethylenediox- ythiophene:polystyrene sulfonate (PEDOT:PSS) charge injection layer and the photoactive layer consisting of poly(3-hexylthio- phene):(6,6)-phenyl-C61-butyric-acid-methyl ester (P3HT:PCBM).
Furthermore,theyevaluatedarangeofparticlesizesandspacingl, whichcanadditionallyincreasetheabsorption.Particlediameterof 10–28nmwerefoundtoimprovethedeviceefficiencyeffectively.
Theoptimumratioofparticlediametertoparticlespacingwasinthe rangeof1.1–3.8forparticlesatthePEDOT:PSS/P3HT:PCBMinterface.
Similarly,Zhuetal.showedintheirmodelingthatparticlesizeand particle spacingof metalnanospheres incorporatedas hexagonal periodic arrays are importantparameters for the performanceof organicPVdevices[47].Theauthorsdeterminedthelightconcentra- tionfactorfor5,10,15and20nmsilverspheresinP3HT:PCBMand PCPDTBT:PCBMdevices.Themaximumlightconcentrationfactor depends on the photoactive layer. However, for center-to-center distancesmuchlargerthantheparticlesizetheconcentrationfactor reducesto1.Theyalsodeducedfromtheirmodelthatevenaverythin spacer/insulatorlayer(<5nm)ontopofanAgnanoparticlestrongly reducesthedegreeoflightconcentration.
Role ofparticlesinPV
Thephoton–excitonconversionrateandtheexcitondiffusionto an interface to dissociate and charge collection are the main limiting factors for the maximum efficiency of PV devices. In particularthin-filmsemitransparentsolarcellssufferfromweak absorptionoflight[10,21,22,48].Herelightmanagementbecomes essentialtoinfluencethephoton–excitonconversion.Theabsorp- tionbecomesoptimalwhenthe reflectionand transmissionare minimal.Scatteringintoandinsidetheactivelayercanenhance the overall absorption, whereas diffuse reflection at the device interfacesshouldbesuppressed.Thishastoberealizedoverthe whole solar spectrum, independent from the polarization and angleofincidence[49].
Figure2(a)showsthesolarspectrumofairmass(AM)1.5[50]and itsintegration,whichisproportionaltothemaximumattainable photocurrent.Inadditionthebandgapsoropticalgapsofdifferent materials,frequentlyusedinPV,arehighlightedasdashedlines(n- type Si [51], P3HT [52],and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta [2,1-b;3,4-b0]dithiophene)-alt-4,7(2,1,3-benzothiadia- zole)](PCPDTBT)[53]).Thespectrumdefinesourregionofinterest forlightmanagementstructures.Colloidshave typicallydimen- sionsbeloworintherangeofthewavelengthofvisiblelightandcan significantlyinteractwiththeelectromagneticradiationbyboth scatteringandabsorption[54].Wewillfirstfocusonscattering,asit isthemostgenericeffectanddominantformostdielectricparticles usedinPVlightharvesting.ForparticleswithdiametersDl/20 thescatteringintensityisisotropicandfollowsal4-dependence (Rayleigh scattering).Hence the scattering intensityof Rayleigh scattererincreases withdecreasingwavelengthl(seecalculation forSiO2inFig.2(b)).ThisisdesiredforPVapplicationsasitmatches thesolarspectrumintheIRandvisibleregion.Forlargerparticles thedistributionofscatteringcenterswithinonescatteringobject becomeimportantandacomplexangular-dependenceisfounddue tointerferenceeffects(Miescattering[55]).Ingeneralincreasing particlesizeofcolloidsleadstoanincreaseoftheintegratedscatter- ingintensity.
FIGURE1
Schematicdepictionoftheparticlelocation(singleparticle)inaPVdevice (a)reducedtotherefractiveindexenvironmentintheparticlevicinity.If n1=n2theparticleisembeddedinahomogeneousrefractiveindexmatrix, whichcanbetheactivematrixofaPVdevice.Fordifferentrefractive indicesn16¼n2theparticleislocatedinornearaninterfaceoftwo materials.Thiscouldbeforexamplethesolidsupport(glass)orthe electrode.ExamplesforexperimentalPVstudieswherecolloidsare embeddedinsuchindexmatricesarelistedinTable1(b,c)showdifferent scenariosforparticledistributionwithinaPVdevice.Randomparticle structurescaneitherconsistofindividualcolloidswithfairlylargeinter- particledistancesorclusteredparticlessuchasdimersandtrimers,which showsmallinter-particleseparationswithintheclustersbutnoorderofthe clustersthemselves.Orderedparticlestructurescanhave1Dor2D periodicitywhenintroducedasamonolayer.Thelengthscalesaredepicted ascenter-to-centerdistancedandperiodicityl.
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Apossibleroutetoenhancethephotoinducedchargegenera- tion rate in a PV deviceis to enhance the path length of the incident photons within the device. This can be realized by implementing sub-wavelength scattering centers at the front and/or the rear of the cell. Here, colloidal particles are very promising sincethescatteringperformancecanbetailoredover abroadrangebymaterialsselection,particlesizeandgeometry.
Anotherimportantparameteristherefractiveindexofthescat- teringobjectinrelationtotheembeddingmatrix.Incaseofindex matchingbetweenthesetwophases,noscatteringwilloccuratall.
Whereasparticlesfromdielectricmaterialsinteractwithelec- tromagneticradiationpredominantlyviascattering,thesituation ismorecomplexformetalcolloidsduetotheircomplexdielectric function.Figure2(b)showstheextinctionefficienciesofspheresof differentmaterials(Silica,Au,Ag,Si)with80nmindiameter.The spectral overlapwith thesolar spectrumisvisiblealthough the originof the extinction is different:For silicathe extinction is simplycausedbyscatteringaspreviouslydiscussed.ForSi,Agand Authe extinction is notonlyinfluencedby scattering butalso absorption, which leadsto distinctpeaks. The most frequently studied examples forcolloids withabsorptionproperties in the relevantwavelengthrangeforPVapplicationsareparticlesofthe noblemetalssilverandgold[56].Whenreducedtothenanosize, suchmetalcolloidsshowstrongabsorptionpeaksduetocollective oscillations of the electronsofthe conduction band. Theseso- calledlocalizedsurfaceplasmonresonances(LSPR)aredetermined byasinglepeakforrathersmallparticlesizes,whichresemblesthe dipolarmode. InFig.2(c)simulatedextinction profilesforgold nanoparticlesofdifferentdiameterfrom40to380nmareshown.
ThesecalculationswererealizedbytheauthorsusingMietheory [57]formonomers,generalizedmultiparticleMietheory(GMMT) [58] for dimers, and finite-difference time-domain (FDTD) [59]
modelingfortheintensityplots.Theplasmonresonanceatlargest wavelengthisalwaysrelatedtoadipolarmode.Additionalhigher modes(quadrupolar,octupolar)areexcitedforlargerparticlesizes [60]. It is also characteristic that the scattering cross-section increaseswithincreasingsize.Thisisdemonstratedbythecalcu- latedscatteringperformanceshownintheinsetinFig.2(c)[26].A balance between scattering and absorbance cross-section is achievedat 80nm.Thus theabsorptivelossplays acrucialrole ifplasmonicparticlesareofinterestasscatteringobjectsinPV[25].
Particle size has also a pronounced effect on the wavelength positionofthe absorptionband.ThepositionoftheLSPRlLSPR asa functionofparticlesizecanbequantifiedby thefollowing scalinglaw(allometricpowerlaw)[61]:
lLSPRðDÞ¼kDaþc (1)
Inthis equation, k is an amplitude parameter, D the sphere diameter(applywithoutunit),athescalingexponentandcisan offsetparameter.Thevaluesfork,aandcwilldirectlydependonthe surroundingrefractiveindex.Forwater(air)assurroundingmaterial theparametersareasfollowed:k=0.0005nm(0.0001nm),a=2.52 (2.63), and c=525.1nm (505.4nm). In general an increase in refractiveindex willcausea plasmonicresonance redshiftalong withanincreaseincross-sectionintensity[61].
Dueto theirplasmon resonance,metalnanoparticles exhibit strong,localizedelectricfields[62].Thesenear-fieldsareofgreat interest for enhancing the photon–exciton conversion in PV TABLE1
Overviewofexemplarystudiesusingcolloidsembeddedindifferentrefractiveindex(RI)environmentsasschematicallyillustratedin Fig.1(a).Theconditions(n1=n2)and(n1n2)standforparticlesembeddedintothephotoactivematrix,whereas(n16¼n2)represents thattheparticlesareattheinterface.
RIenvironment Reference Particlelocation Device
n1=n2 Baeketal.[30] AgnanoparticleinPEDOT:PSSlayer Organic
Fuetal.[31] AunanoparticleinPEDOT:PSSlayerorin hybridPCPDTBT:CdSelayer
Organicandhybrid Gangishettyetal.[32] Au@SiO2nanoparticleinTiO2-matrix Dye-sensitized
Luetal.[33] AuandAgnanoparticleinPEDOT:PSSlayer Organic
Pacietal.[34] AgnanoparticleinP3HT:PCBMlayer Organic
Qietal.[35] AgandAg@TiO2nanoparticlesinTiO2
photoanode
Dye-sensitized
Spyropoulosetal.[36] AunanoparticleinP3HT:PCBMlayer Organic
Yangetal.[28] AunanoparticleinPEDOTlayer Tandem
Wangetal.[27] AunanoparticleinP3HT:PC70BMlayer Organic
n1n2 Sheehanetal.[37] Au@SiO2andAu@SiO2@TiO2nanoparticle
inphotoactivematrix
Dye-sensitized Standridgeetal.[38] Agnanoparticleinphotoactivematrix Dye-sensitized Woohetal.[39] Au@SiO2nanoparticleinphotoactivematrix Dye-sensitized
Xuetal.[40] Alnanoparticleinphotoactivematrix Dye-sensitized
n16¼n2 Grandidieretal.[41] SiO2particleonfront(ITO/airinterface) a-Si:H
Pastorellietal.[42] AunanoparticleatinterfaceITO/TiO2layer Organic Choietal.[29] Au@SiO2atinterfaceITO/PEDOT:PSSandat
interfacePTB7:PC70BM/PEDOT:PSS
Organic Brownetal.[43] Au@SiO2atinterfaceTiO2/hole-transportlayer Dye-sensitized Kawawakietal.[44] AunanoparticleatITO/TiO2interface Dye-sensitized Yoonetal.[45] Agnanoparticleatinterface
PEDOT:PSS/photoactivelayer
Organic
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devices[63,64].Henceitisoftendesiredtocontrolthestrength andrangeofthesefields.Themagnitudeandthenumberofthe near-fieldresonancesisstronglydependentonthetypeofmetal, the size and shape of the particle as well as the inter-particle distance:Forincreasingparticlesizethestrengthofthenear-field and its spatial dimensions rise. The number of resonances is reverselyproportionaltothe symmetryaxesofthe particle.For instance,nanospheresonlyshowoneresonance,whereasnano- cubeshavelowersymmetryaxisandshowup tosixresonances [65,66].Figure2(b)showstheextinctionefficienciesofspheresof different materials (Silica, Au, Ag, Si) with 80nm in diameter.
Whileforthesilicasphereonlylocalfieldintensitiesupto2.5are calculated,thevaluesforgoldandsilverspheresareapproximately tentimeshigher. Furthermorethisenhancementcanbesignifi- cantlyincreasedwhenparticlesarebroughtintoclosevicinityto eachother.AscanbeseenfromtheresultsofFDTDsimulationsfor apairofsilverspheresshownintheinsetgraphicinFig.2(d),the electricfieldisthehighestintheparticlegap.Theopticalinterac- tion between approaching colloids will be treated in detail in section‘Localandcollectivecouplingeffects’.
Thequestionofhowtheincorporationofplasmonicparticlesis actuallyenhancingtheperformanceofthinfilmsolarcellsisstill underdebate.ItcriticallydependsonthedesignofthePVdevice andtheobservedenhancementhasbeenattributedtolightscat- teringattheparticles[27],orcontributionsfrom(plasmonic)near- fieldenhancement[30]orboth.Furthermore,metallicparticleson topofaSisolarcellcanactasantireflectivecoatings[67].Wang etal.investigatedthedifferencebetweena10nmand70nmgold nanoparticle,whichwereincorporatedintothephotoactivelayer.
TheAucolloidsnotonlyincreasedthelightabsorptionvialight scattering, but alsoimproved the charge transport throughthe activelayer.Remarkably,metallicparticlesurfaceshavenotbeen passivated,whichisusuallydonetopreventexcitonquenching andunwantedrecombination[35].Baeketal.immobilizedarange ofdifferentlysizedAgnanoparticlesintheholeconductinglayer athighvolumefractions(40%)[30].Theyfoundaclearcorrelation betweentheAgnanoparticlesize,itsplasmonicresonanceandthe maximumexternalquantumefficiency(EQE)ofthedevice.Fora PCDTBT/PCBM based solar cell Ag nanoparticle of 67 nm in diameter featuredthe highestenhancementof 13%at a power FIGURE2
Simulatedextinctionfordifferentcolloidalsystemsforabsorptionandscatteringenhancement.(a)Solarspectrumandintegratedspectralradiancewhichis proportionaltothephotocurrent.(b)Extinctionefficiencyandenhancementfactorofadielectric(silica),ametalloid(orsemiconductorc-Si)(silicon),and metallicnanoparticles(goldandsilver)of80nmdiameter.(c)Extinctionefficiencyofgoldnanospheresinairwithdifferentparticlediameterandscattering performance(shownininsetfordifferentnanoparticles).(d)Extinctioncross-sectionofananoparticledimeratvariousinter-particledistances(spacing).
Electricfieldintensityplotsimagedatresonancefrequency(unitsjEj2/jE0j2).
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conversionefficiency(PCE)of7.6%.Forthisnanoparticlesizethe authorshavealsoinvestigatedtheinfluenceofparticleconcentra- tiononthe deviceperformance.Aconcentrationof4.5109par- ticles/cm2wasfoundtoyieldanoptimuminPCEandshortcircuit currentdensity(Jsc).Forhigherconcentrations,decreasingJscand fillfactor(FF)valueswereobserved.Thisreducedperformancewas relatedtotheformationofparticleclustersduringthedeposition ofhigheramountsofparticles.Thisexamplenicelyillustratesthat notonlyparticlesizeisanimportantparameterforcolloidallight managementinPV,butalsotheparticleconcentrationandparti- cleseparationarecrucial.
Aninterestingalternativetoinfluencetheabsorptionandscat- tering of plasmonic nanoparticles overa broadrange of wave- lengthisgivenbythepreparationofhollowmetalnanoshells.The opticalpropertiesofsuchnanoshellsaredirectlydeterminedby the nanoshell dimensions. These hollow metalstructures have veryhighextinctioncross-sectionsbutstillshowsufficientcolloi- dalstabilitycomparedtometalspheresofsimilarsize.Theabsorp- tionand scatteringbehavior ofthe nanoshellscanbe precisely tuned by variation of their outer and inner diameter [68,69].
Recently Paz-Soldanet al.have employedgoldnanoshellswith finelytunedspectralproperties(scattering-to-absorptionratio)to enhancetheperformanceofquantumdotsolarcellsinaspecific spectralrangewherethecellperformanceisratherlow[70].The authorshavecombinedexperimentalandtheoreticaldescriptions tofindtheidealmatchinspectralpropertiesoftheplasmonicand theexcitonicmaterial.Ascomparedtonon-plasmonicreference cellsaPCEenhancementof11%wasfound.Inthenear-infrareda peakenhancementof35%wasmeasured.Thisexampleshowsthat rational design of colloidal particles with well-defined optical properties is a promising way to enhance the performance of PVdevices.
Fromtheexamplesandconsiderationsprovidedinthissection it becomes apparent that colloidalbuilding blocks inherently possess ahugepotentialto influence lightmanagement in PV devices.Themain factors arescattering, absorption,andlocal fieldenhancement,whichintricatelydependonthetypeofthe particle aswellasitssurroundingmedium. Theireffect onthe electricalperformanceaswellasreflectionandtransmissionat theinterfacesmayalsonotbeneglected,though.Sinceparticle composition,sizeandshapeareofutmostimportance,thefol- lowingsectionwillfocusonsyntheticapproachesforcontrolling theseparameters.
Particledesign:size,shape,material
Nowadaysaplethoraofcolloidswithtailoredopticalpropertiesis availablesincethenumberofworksdealingwiththesynthesisof colloidal systemswithcontrolled composition,particlesize and particle shape has steadily increased during the last decades.
Choosing the ‘right’ particles for PV applications is complex because the ideal composition, size and shape will depend on thedesiredtypeofenhancementeffect(e.g.scatteringornear-field enhancement)aswellasthedevicelayout,amongstotherparam- eters suchaschemicaland electrical compatibility. Upto now mostoftheexamplesintheliteraturewherecolloidshavebeen employedaslightmanagementstructuresinPVapplicationsfocus on particles which are nearly monodisperse in size and shape.
Hencetheparticlesusedhavewell-definedscattering,absorption
andnear-fieldproperties.Thusopticalpropertiescanbeprecisely tailoredtomatchforexampletheextinctionoftheactivelayer.In addition,theoreticalsimulationsaremucheasiertobeperformed foramonodispersesystem.However,dependingontheenhance- menteffectandthetypeandlayoutofthedevice,colloidswith polydispersityinsizeandshape,andevenincompositionmayalso be well suited for application in PV, although a quantitative correlationwithdeviceperformancehasnotbeenrealized.Nev- erthelesstherearemanyexamplesintheliteraturewhichclearly requiremonodsipersecolloidsasforexamplewhentheseareused asphotonic bandgapelements. Inparticular,colloidal polymer particlessuchaspolystyreneorpolymethylmethacrylateareof relevance because they are rather cheap and easily available throughemulsionpolymerization[71,72].Thismethodcanpro- videcolloidallystablepolymerparticleswithsizeswhichcanbe controlledbytheamountofsurfactantemployedinthepolymer- ization.Othercommonapproachesforthe preparationofsuch particles are miniemulsion [73,74] and emulsifier-free [75,76]
polymerizations.Figure3(a)showsmeasuredandfittedscattering cross-sectionsofpolystyreneparticles ofvarioussizes.Forsmall sizesthescatteringdatacanbewell-describedasRayleighscatter- ing.ForlargerparticlesizesMiescatteringisobserved.Thescat- teringcross-sectionincreasessignificantlywithincreasingparticle size.Inaddition the spectral rangewhereeffectivescattering is observedchangeswithincreasingdiameter.Thus,polymerparti- cleswith dimensions, comparable to the wavelengthof visible lightarepromisingcandidatesasscatteringcentersandasbuilding blocksforphotonicstructuresinPV.Apartfrompolymer based materials, silica particles have been shown to be suitable for efficiencyenhancementinPVdevices[77,78].Monodispersesilica colloidscanbepreparedbythewell-knownprotocolofSto¨beretal.
[79].Thismethodreliesonhydrolysisandsubsequentcondensa- tionofasilicaprecursorinalcoholsunderbasicconditionspro- vidingparticleswithdiametersrangingfrom50nmto2000nm canbeachieved.Cheaperapproachesareavailableifmonodisper- sityislessimportant[80].
Whileaclearadvantageofsilica-andpolymer-basedmaterialsis thelowcostandeasypreparation,theratherlowscatteringcross- section(compareFig.2(b))limitstheiruseinPVapplications.The scatteringperformancecanbesignificantlyalteredifhollowsilica spheresareprepared. Figure 3(b) shows photographsand back- scatteringspectraofhollowsilicaparticleswithdifferentoverall diameterbutratherconstantshellthicknesses[81].Miescattering inthevisiblewavelengthrangeisobserved.Theeffectiverefractive index of such hollow structures is determined by the particle diameter, shell thickness, and the shell material. Expectedly, the scattering cross section drops with decreasing aspect ratio betweenshellthicknessandparticleradius.However,thetrans- portmeanfreepathoflight increasesconsiderablyatthe same timeinsuchdryparticlepowders.Interestingly,whenembedding hollowspheresintoamatrixmaterial,thehollowcorewithitslow refractiveindex(n=1.0)canthenactasstrongscatteringcenter, duetothehighcontrasttothesurroundingmedium,whichfor mostpolymersisaroundn=1.5[61].
Probablythemost frequentlyappliedcolloidalsystemsinPV devicesaregoldandsilvernanoparticles.Asimpleroutetoprepare goldnanoparticlesofabout20nmindiameteristhewell-known citratereductionprotocol by Turkevichet al.[82]This method
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FIGURE3
Tunablelightmanipulationwithnon-absorbing(a,b)andabsorbing(c,d)colloids.(a)SizedependentRayleigh(1)andMie(2,3)scatteringofpolystyrene spheres[108].Thescatteringcrosssectionincreaseswithincreasingparticlesize,wherebythelargestspheresshowtypicalMietypeIIscatteringoscillations atshorterwavelengths.(b)Singleparticlespectraofhollowsilicananoparticlesobtainedbyconfocaldarkfieldspectroscopy[81].TheinsetsshowSEM imagesofhollowsilicaspheresandthevisibleMiescatteringofvialswithdifferenthollownanoparticles.Thescalebarscorrespondto200nmandthe numbersontopofthevialsindicatetheoverallparticlediameterandshellthicknessinnm,respectively.(c)Sizecontrolledabsorbanceofsphericalgold nanoparticles[90].Increasingtheparticlesizeresultsinaredshiftofthelocalizedsurfaceplasmonresonance.TworepresentativeTEMimagesofthehighly monodispersesphericalparticlesareshown.(d)Goldnanorodswithfinelytunedplasmonresonancespreparedviathermalreshaping[103].ShownareUV- VisabsorbancespectraandrespectiveTEMimagesafterdifferenttimesofthermaltreatment.ß(a)AIPPublishingLLC,(b–d)AmericanChemicalSociety.
7
RESEARCH:Review
yields nearly spherical, polycrystalline particles, which show a dipolarLSPRataround520nm.Frensetal.havestudiedtheeffect oftheconcentrationofthereducingagentcitratesystematically andshowedthatparticlesrangingfrom12to150nmindiameter canbeprepared[83].SimilarlyKimlingetal.haveexaminedthe reactionconditionsusingcitrateandascorbicacidandfoundthat narrowlydistributedparticleswithsizesrangingfrom9to120nm areaccessible[84].Whiletheseprotocolsyieldgoldparticlesoflow polydispersitywithsizesrelevantforPVapplications,thevolume fractionislowinallofthesemethods.Lietal.haverecentlyshown thatthe achievableconcentrationcanbesignificantlyincreased (about 5 times higher) using sodiumhydroxide and controlled reactiontemperatures[85].
Smallergoldparticlescanbepreparedusingstrongerreducing agentsbutareoflessrelevanceforPVapplicationsduetotheir weakscattering/absorption.Incontrast,largerparticlesizesare often required. Awell-established route toward larger particle sizesisbasedonseed-mediatedgrowth.Theadvantagesofthis procedurearemonodisperseparticlesandthepossibilitytofine tuneparticlesizebytheratio seeds/metalprecursor[86–88].A disadvantage of theseseed-mediated methods is the typically largeamountofsurfactantusedduringthegrowthofparticles.
Thus,excessivecleaningisrequired,whichincreasesthecostof the material.Niu et al. haveshown a one-step seed-mediated synthesis,withoutusinglargeamountsofsurfactantsforstabili- zation,basedongoldnanoparticleseedspreparedbythemethod ofFrens[89].Theauthorshaveused2-mercaptosuccinicacidas reducingagent intheseededgrowthofparticlesuptosizesof 150nm.SolelyusingcitrateBastusetal.reportedrecentlyona modifiedcitratereductionroute,whichallowsforgenerationof particlesofdifferentdiameter[90].Figure3(c)showstworepre- sentativeTEMimagesofnearlysphericalgoldnanoparticlesalong withUV–visabsorbancespectraofaseriesofgoldnanoparticles withincreasingdiameterpreparedbythismethod.Aclearred- shiftoftheLSPRwithincreasingparticlediameterisobserved.For largerparticle sizes,additionalplasmonmodesappear andthe absorbanceislargeoverabroadrangeofwavelength.However, theplasmonresonanceofgoldisstronglydampedwhichleadsto rather highabsorptivelossesanda limitednear-field enhance- ment[91].Whenconsideringthenear-fieldenhancement,silver nanoparticlesaremorepromisingcomparedtogoldnanoparti- clesbecausetheirLSPRislessdamped.ForPVapplicationssilveris also a much more interesting materialdue to its significantly lowerpricecomparedtogoldbutbarestheriskofoxidation.Very similartotheFrensmethodforpreparinggoldnanoparticles,Li et al. have synthesized nearly monodisperse, spherical silver nanoparticlesusingcitrateandascorbicacidinthepresenceof iodideions[92].Thiswaytheauthorscouldpreparesilvernano- particleswithsizesof20–40nmstabilizedbycitrate.Morerecent- lyLietal.havereportedonthesizecontrolledsynthesisofsilver nanoparticlesfromdifferentsilverprecursors[93].Theparticle sizecouldbecontrolledin adiameterrangeof16–30nm.The dipolar LSPR of these particles are located around 400nm.
ExtendingthediameterrangeEvanoffetal.havepreparedsilver nanoparticlesofnearlysphericalshapeinthesizerangeof15–
200nmbythereductionofsilver(I)oxidebyhydrogengas[94].
Thesameauthorshavealsostudiedtheextinction,scatteringand absorption cross-section of these particles in detail [95]. The
smallerparticlesshowedonlydipolarLSPRmodesatwavelength around400nm.Exceedingaparticlediameterofaround90nm,a quadrupolarcontributionappearedcloseto420nmwhereasthe dipolarmode shiftedtowardhigher wavelengthfor increasing particle size. Bastus et al. have reported on the size selective synthesisofsilvernanoparticleswithdiameterrangingfrom10 to200nmstabilizedbycitrate[96].
The near-field and far-field optical properties of gold and silvernanoparticlescan besignificantlymanipulatedbyintro- ducingnon-spherical symmetry. During thelastdecade many protocols have been published allowing for the synthesis of noble metal nanoparticles with non-spherical shapes. Shape controlofnoblemetalcolloidsisachievablethroughseedme- diatedgrowth.Thistwo-stepstrategyallowsforthesynthesisof well-defined colloids through the separation of nanocrystal nucleation and selective growth of individual facets. At first, uniform seed particles are synthesized through reduction of metalionsinthepresenceofasuitablestrongreducingagent.
Inthesecondstepmetalionsandshapetemplatingmoleculesor surfactants are addedwhichcan resultin non-spherical over- growth. This way for example particles with rod-shape [97], triangular shape [98], nanodecahedra [99], nanostars [100], and nanocrystals with defined platonic shapes [101] can be synthesized. Particularly interesting are rod-shaped plasmonic nanoparticlesdueto theirwell-pronouncedresonances,which arepolarizationdependent[102].Collectiveoscillationsofthe valenceelectronsalongthelongaxisofsuchrodscauseavery intense longitudinal plasmon resonance. The position of this resonancecanbefinelytunedoverabroadrangeofwavelength byadjustingtheaspectratiooftherods.Thiscanbeachievedfor example either by thegrowing conditions[97] orby thermal decomposition[103].Figure 3(d)showsabsorbancespectra for goldnanorodsofdifferentaspectratiosobtainedafterdifferent timesofthermaltreatment.Theyshowtwodistinctabsorption features:Thetransversalmodeat around520nmandthelon- gitudinalmodeatlongerwavelength.Reducingtheaspectratio ofthenanorodsleads toapronouncedblueshiftofthelongi- tudinalresonancewhereasthetransversalmoderemainsnearly unaffected.Hencetuningtheaspectratioofrodsisapowerful meanstocontrol theabsorbanceproperties. Formoredetailed informationonsizeandshapeselectivesynthesisofnoblemetal nanoparticles the reader is referred to the following reviews [54,104–107].The conceptof broadband plasmonicresonance tuning has been nicely demonstrated by Lu et al. [33]. They embedded40nm– 50nmAuorAgnanoparticlesinthePED- OT:PSSlayerandachievedthehighestPCEof8.67%forablend ofbothparticletypes,whichfeaturedplasmonicresonancesat 425nmand529nm,respectively.Theyexpandedthisconcept furtherbytheuseofAurodsof40nm–50nm inlength and 15nminwidth,whichtranslatedintoplasmonicresonancesat 515nm(transversal)and679nm(longitudinal).Theabsorption enhancementintheEQEspectraresultedinahighPCEof8.41%
-comparablyhightotheAg/Aublend.
These examples of colloidal design extent their multiple influenceson light management in PV devicesas outlined in section ‘Role of particles in PV’, wherescattering, absorption and local field enhancement were key, to multicolor photon management.
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RESEARCH:Review
Whiletheprevioussectionhaspresenteddesignroutestoward synthesis of particles with well-defined optical properties, the integrationoftheseparticlestypicallyrequiresadditionalsurface functionalization,inordertoensurecolloidalstabilityaswellas compatibility with the matrix material and controlled particle distribution/surface coverage. Thus the following section will introduceconceptsforsurfacefunctionalizationofparticles.
Particlefunctionalization
Colloidalparticlesaretypicallymetastablesinceinthermodynam- ic equilibrium aggregation of particles due to van der Waals interactionis favorable. This aggregation leads to a significant changeor evenlossofthesingleparticlepropertiessuchasthe well-definedlocalizedsurfaceplasmonresonances.Furthermore, thestability ofcolloidsplays amajorroleforparticleassembly, whereuncontrolled aggregation usually needsto be prevented.
Thus,thecolloidalstabilityisacrucialaspectforthepreparationof colloidal light harvesting structures with well-defined optical properties. Stabilization requires repulsive interactions. These canbeelectrostatic interactions,whichstronglydependonthe ionic strength, and/or steric stabilization. Increasing ionic strengthcanbeusedtolowertherepulsiveelectrostaticinterac- tions.Thiswaytheparticle-particleseparationupon particleas- semblycanbereduced.Ithasbeenshownthattheadditionofsalt canbeusedtoobtainmonolayersofgoldandsilvernanoparticles withcontrolledinter-particledistancesinsolarcelldevices[12].In this work a balance between high surface coverage and well- definedseparationbetweenindividualparticleswasachieveden- ablingplasmoniclayerwithcharacteristicextinctionproperties.
Apartfromelectrostaticrepulsion, stabilityinmanycolloidal systemsreliesonstericeffects.Stericstabilizationtypicallyrelies onacoronaofchainsattachedtoa nanoparticlesurface.These chainspreventdirectcontactbetweenthesurfacesofneighboring nanoparticles.Increasingthechainlengthofthestabilizermoie- ties can be employed to enhance stability and to increase the averageparticle-particleseparation.Toacertainextentthiscanbe usedtotuneparticle-particleseparationsinorderedparticlearrays.
Manywet-chemicalsynthesisprotocolsprovideparticleswhich arealreadystabilizedeitherbychargedfunctionalities(e.g. car- boxyl-,sulfate-,sulfonate-,amine-andamidine-groups),byrather bulkyandflexiblemoietiesprovidingstericstabilizationorbya combinationofboth(electrostericstabilization),becausecolloidal stabilityduringsynthesisiscrucial.Thisfunctionalizationhowev- erisinmostcasesnotatalloptimizedforthestructuralrequire- mentsofsubsequentassemblyorforintegrationintoPVsystems.
Thus changing the particle surface by ligand exchange or by growingashellaroundthecolloidsisrequired.Ligandexchange allowstailoringthepolarityoftheparticles,whichcanbeusedto performphasetransferfromonesolventtoanotheroroptimize themintheircompatibilitywiththePVmatrix[111,112,126–128].
Bothshellgrowth and ligandexchangeallow controllinginter- particledistanceswithrespecttoPVapplications.Dielectricshells areideallysuitedsincetheydonotsignificantlyaltertheoptical propertiesoftheencapsulatedcolloid.Suchshellsideallycoatthe colloidshomogeneouslyandpossessa definedthickness.When twoofthesecore-shellcolloidsarebroughtintocontacttheshell canactasaspacerandthespacingdsofthecolloidalcoreswillbe twicetheshellthicknessincaseofnon-deformableshells.Agood
example forthis aresilica-coated goldnanoparticles [109]. The silicashellscauseonlyslightchangesoftheopticalpropertiesof thegoldcoresbutpreventdirectcontactofthegoldparticlesin assembledstructures, whichwould significantlychangethe ab- sorptionoftheparticlesduetoplasmonresonancecoupling[110].
Whilethesilicacoatingstrategyhasthe advantageofthickness controlonthenanometerorevensubnanometerscaleandithas beenadoptedtocoatalsoforexampleanisotropic goldorgold- silvercore-shellnanoparticles[111,112],andsphericalsilverpar- ticles [29,113],adisadvantage isthe factthat the protocolsare usually performedwith step-by-stepgrowingofthe silica shell.
Thismakestheovergrowthprocedureelaborateandtimeconsum- ingifthickshellsaredesired.Polymerbasedmaterialsmayhave severaladvantageswhenusedascoatingmaterial.Figure4givesan overviewofinorganicnanoparticlescappedbydifferentorganic materialsenablingawide rangeofparticle spacings.Very small particleseparationsdscanbeachievedwhensmallcappingmole- culessuchascitrate (a)[114]oralkylammonium molecules(b) [115]areused.Inthiscasevaluesofdsintheorderof1nmcanbe achieved.Thisisaseparationwhichistypicallymuchsmallerthan thediameterofthenanoparticles.TheTEMimagesclearlyshow that direct contact between the gold nanoparticle surfaces is avoided due to the presence of the shell. Other examples to achievesuchsmallseparationsarealkylligands[116–118],DNA [107,119–122],orshort-chainPEG-ligands[123],amongstothers.
Utilizing larger molecules as ligands gives access to larger ds. Figure4(c,d)showsexamplesofparticleseparationsintheorder of10nmachievedbypolystyrene(c)[124]andDNA(d)[125].A furtherincreaseindsispossiblebyincreasingthemolecularweight ofthestabilizingpolymerasshownforlinearpoly-N-isopropyla- crylamide[126]inFig.4(e).Evenlargerparticleseparationspro- viding assemblies with lattice constants in the range of the wavelengthofvisiblelightareshowninFig.4(f).Hereanexample forhydrogel-coatedinorganicnanoparticles(silica)isshown.The concept ofhydrogel-encapsulationhas beenexploitedforsilica [127,128],gold[129,130],andsilver[131]nanoparticles,amongst othermaterials.
AllexamplespresentedinFig.4showthatnanoparticlescanbe well-separatedbyintroducingcappingmoleculesonthenanopar- ticle surface. The range of possible capping species and their anchoringgroupsislargeandthereexistmanymoresystemsas presented in Fig. 4. Neverthelessthe examples in Fig. 4 nicely demonstrate thatseparationscoveringmorethantwo ordersof magnitude beginning at nearly atomicdimensions arepossible usingorganicshells.
Once the substrate is covered with colloidal particles with specific inter-particle spacing, surface coverage determines the opticalproperties,thatis,absorption,scatteringandtransmission [132–135].Whereasalowareafractionfareaofonlyafewpercent willonlyhavenegligibleinfluence,ahighareafractionwillfor examplesignificantlylowerthetransmissionthroughthislayer.
The maximum area fraction for monodisperse spheres (close- packing)canbecalculatedasfollows:
farea¼ pR2 2R
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3=4ð2RÞ2
q ¼ p
2 ffiffiffi
p ¼3 0:91 (2)
Here,Risthesphereradius(R=D/2).Itisobviousthatcontrol- ling the surface coverage is one of the key challenges for the
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RESEARCH:Review
implementationofcolloidsinPV.Besidesthedirectinfluenceof thesurfacecoverageontheabsorption,scatteringandtransmis- sionproperties,thecostofthecolloidalmaterialwillrequirethe useofthe leastamountofmaterialaspossible.Thus,a tradeoff existsbetweenreducingtheamountofcolloidslimitingproduc- tioncostsbutatthesametimetoimplementenoughmaterialto achieveanopticaleffect.
Furthermore,theopticalperformanceofacolloidalmonolayer isstronglyinfluencedbytheinter-particleseparation,thatis,the spacingbetweenneighboringcolloidsds.The spacingdscanbe directlycorrelatedwiththesurfacecoveragefora2Dmonolayer withhexagonalpacking:
ds¼2R
ffiffiffiffiffiffiffiffiffiffi 0:91 farea s
1
!
(3)
For example a perfect hexagonalmonolayer of spheres with R=50nm andanareafractionfarea=0.5willhavea spacingof ds=35nm. Therearein principletwo waysto controlthe area fraction ofparticles in a monolayer: (1) throughthe design of colloidal buildingblocks,that is,introducingligands/shellssur- roundingtheparticles.(2)Throughtheassemblyprocess.
Apart from the optical performance, it is also important to considertheelectricalandchemicalchangesofaninterfacewhen a colloidal monolayer is applied. Otherwise, for example, the consequencesforthechargetransportcanbesevere.Yoonetal.
depositeda monolayerof 4nm Agnanoparticles rightbetween PEDOT:PSSandthephotoactivelayer[45].Indeed,anenhanced absorptionwasobtained and theJscincreasedaccordingly from 6.2mA/cm2(noAgnanoparticles)to7.0mA/cm2.However,the fillfactorandopen-circuitVoltage(Voc) decreasedsignificantly, whichresultedoverallinareducedPCEof1.2%comparedto2.2%
ofthereferencecell.Thisclearlydemonstratestheneedtoadjust opticalandelectricalpropertiessimultaneouslyinordertoachieve anoverallimprovement.
Summarizing,controlledsurfacemodificationisessentialfor compatibilization of particles with the PV environment and even allowscontrolling inter-particledistances and thus gives ahandleontailoredsurfacecoverage.Whereasopticalproper- ties of randomly distributed particles with large inter-particle distancesareadditive,additionaleffectsemerge,iftheparticles are distributedin a periodic fashion and/orshowsmall inter- particle separations. This will be discussed in the following section.
FIGURE4
Differentstrategiesofcontrollinginter-particledistancedsbetweencolloidalparticles.(a)Aunanoparticlesstabilizedwithcitrateleadingtoaninter-particle separationmuchsmallerthantheparticlediameter[114].(b)Aunanoparticlescoatedwithtrialkylammoniumbromideprovidingparticleseparationsinthe orderof1nm[115].(c)Particleseparationof7.2nmobtainedthroughligandexchangeonironoxidenanoparticles(about5nmindiameter)using polystyreneligands[124].Thespacingbetweenparticlescanbeincreasedbyusinghighermolecularweightpolystyreneligands.(d)DNA-protectedgold nanoparticleswithapproximately10nminter-particlespacingandfastFouriertransform(FFT)asinset[125].(e)Linearpoly-N-isopropylacrylamideligands ongoldnanoparticlesgiveaccesstoparticleseparationsintherangeof12–41nm[126].Largeparticleseparation,intheorderofthewavelengthofvisible light,obtainedthroughcross-linkedhydrogelshells.Particlespacingsintheorderof200nmwereobtainedforsilicananoparticles(about70nmin diameter)coatedwithcross-linkedpoly-N-isopropylacrylamide.NotethattheorganicshellsinallTEMimagesshownherearealmostnotvisibleduetothe ratherlargedifferenceincontrastbetweentheinorganiccoresandtheorganicshells.ß(a,b,f,e)AmericanChemicalSociety,(c)JohnWileyandSons,(d) NaturePublishingGroup.
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RESEARCH:Review
Localand collectivecouplingeffects
Beyondtailoringopticalpropertiesofparticlesonthesingleparticle level,couplingeffectsbetweenmultipleparticlesopenanexciting way of tuning scattering and absorption. Here, we distinguish betweenlocalcouplingeffectsandcollectiveeffectsthatrequire longrangeorder.Forsimplicity,wediscussonlysphericalcolloidal particles,buttheconceptscanbeextendedtoparticleswithless symmetryaxessuchasrodsorcubes[136,137].
Bringingtwoparticlesincloseproximityhasasignificanteffect onthe particles scattering performanceand their near-field be- causeoflocalcouplingeffects.Figure5(a)showsaschematicofa dimerassemblywiththerefractiveindexofthesphere(ns),sphere diameters(D1,D2),andinter-particlespacing(ds)locatedontopof aPV.Therefractiveindexofthesphereshouldbechosendiffer- entlytotheenvironmenttohavecontroloverscattering,absorp- tion, and enhancement. A simple way to cover the full solar spectrum and to avoid strongpolarization effects is to choose differentdiameters ofthe spheres (D1>D2) or to assemblethe colloids into more complex structures like trimers. The inter- particle spacing can be controlled experimentally over a wide distance range with particle surface modifications as has been shown in the previous section ‘Particle functionalization’. The blueshiftoftheplasmonicresonanceinFig.2(d)followsauni- versalscalinglaw calledthe plasmonrulermodel, whichisap- proximately proportional to an increase of the inter-particle spacingscaledby thediameter [138].Dueto the lownear-field enhancement for particles from dielectric and semiconductor materials, additional collective light management is important togivecontrolovertheabsorption.
Collective coupling effects are frequently exploitedfor light trapping.Themostimportantlighttrappinggeometriesaresum- marized inFig. 5(b),which areaddressedforspherical particles (refractiveindexparticle,ns:dielectric,semiconductor,ormetal- lic). Initially, we discuss the three light-trapping principles dependingonthelocationoftheparticleassembly(inthefront, embeddedinto,oronthebackelectrode)bythespecificchoiceof the layerrefractive indexn1,n2,andn3and thepositionofthe colloidsinside the opticalsetup: First,lighttrappinginside the activelayer(n1=n2),whichresultsinaclosecontactoftheexcited near-fieldtotheactivelayer[139].Withhomogenoussurrounding refractiveindicesthenear-fieldintensityisequallydistributedas showninFig.2c.Second,theparticlescouldbeusedasscattering elementswitharadiationpatternpointingtowardtheactivelayer (n1=air,n2=activelayer) [24,140].Third, lighttrappingcanbe achievedbyacorrugatedorperiodicallystructuredbackelectrode (n1=n2:activelayer,n3=ns,dL=0)[141–143].
Intermsofthenatureofthecollectiveopticalresonance,the following scenarios are predominant. The colloidal building blocks and the active layer must have a high refractive index (ns=n2>n1: semiconductor materials) in comparison to their environment (n1: dielectric) and must have a back mirror (n3: metal)[21].InFig.5(b)weshowthefourmostcommonmodes, whichcouldenhancelightabsorptioninthespectral regionfor solarcellapplications:(1)aFabry–Perotstanding-waveresonance [10,21,144],(2)guidedresonanceofthesemiconductormaterial [145–147], (3) grating coupling along the periodic structure [148,149],and(4)Whisperinggallerymodewhenawavelength- scaledielectricsphereisinclosecontacttoahigh-indexsubstrate [77,150].Thesemodesoracombinationofthem,whichleadsto hybridization,canoccurinthinfilmsolarcells.Particularlyim- portantareFabry–Perotresonances,guidedresonancesandgrating couplingstructures,fortheuseinbroadbandabsorptionapplica- tions.Theymayalsobeemployedinsituations,wheretheincident light covers a wide angular range relativeto the PV device. A further promisingroute toenhance theopticalabsorptionisto separate the metallic particles with a thin dielectric layer (n2, dL=3–10nm)fromthebackelectrode(ns,n3:metallic),inanalogy tothelighttrappingbyperiodicstructures.Thismetal–insulator–
metalsystemgivescontrolovertheelectricandmagneticcompo- nenttoformanidealabsorber[151,152].
Inordertoachievethestructuralfeaturesnecessaryforthelocal and collective effects mentioned above, excellent control over inter-particle spacing and/or long range order are required. In thefollowingwereviewmethodsthatallowforlargeareafabrica- tionofsuchorderedstructures.
Light absorption enhancement with colloidal assemblies is possiblewithasimplearrayfabricationmethod, whichiseasily scalable withoutthe needof lithography[153].Movingfrom a randomparticlemonolayertoamonolayerofhighsymmetrysuch asthehexagonalmonolayer schematicallydepictedinFig.1(c), stronginteractionwithlightinthevisibleorIRregionoccursifthe periodicitylisintheorderofthewavelengthoftheincidentlight duetogratingdiffraction.Furthermore,thesphericalsymmetryof thecolloidsmakesitnaturallypossibletoacceptalargeangleof incidence.RecentstudiesfromPastorellietal.madeafirstattempt intousingcolloidalassembliesrandomlyalignedwith1nmspac- ing,whichshowanimprovementoftheirshortcircuitcurrentby
4
1 2
θ 3
n
1n
2>n
1n
3Top view
Cross-s ec on
⊗ k
Polari za on
n
sk
d
Ld
sD
1d
sD
2Dime r Trime r
(a)
(b)
FIGURE5
Schematicsofcolloidalthinfilmsolarcelllightmanagement.(a)Dimerand trimercolloidalassembliesforbroadbandlightabsorption(D:sphere diameter,ds:inter-particlespacing,ns:refractiveindexofparticle,k:wave vector).(b)Light-tappingprinciplesandcollectiveopticalresonances,such as:Fabry–Perotresonance(1),guidedresonance(2),gratingcoupling(3), andWhisperinggallerymodes(4).Refractiveindicesofthedifferentlayers:
n1:dielectric,n2:activemedium,n3:backelectrode(ds:inter-particle spacing,dL:layerthickness).ForPVapplicationopticalresonancesshould coveradditionallyabroadangleofincidence(u).
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RESEARCH:Review
morethan12%[42].Astepfurtherwasdonebyusingassemblies with differentparticle sizes(core-satellite plasmonic nanostruc- ture),whicharelinkedwithaDNA-directedself-assemblyprocess onthesubstrateasZhengetal.showed[154].Consequently,the assemblyofdifferentparticlesizes(core-satelliteplasmonicnano- structure) or particle cluster assemblies [155,156] (trimer in Fig.5(a)withconstantsize)areofparticularimportanceforlight absorptionof a broadfrequencyrange and forthe polarization independencebecauseoftheirstructuralsymmetry[49].
Figure6(a)showsanexperimentalapproachforaparticleline assembly,whichisfabricatedwithalithographyfreestamptech- nique[157–159].Thistemplateassistedassemblymethodmakesa centimeterscaledalignmentpossiblewithadefinedinter-particle spacinganddefinedorientationoftheparticles.Furthermore,the modification ofBSAcoating (Fig. 4) allowedanultra-small gap spacing(<2nm)withalowdefectrate.Thislargeareaprecision couldbeseeninthe opticalresponse andin thecorresponding modeledspectra.Dependingonthepolarizationagreatdiversity
FIGURE6
Excitationofopticalresonancesforperiodicallyarrangedcolloidalparticles.(a)Experimentalrealizationofnanoparticlechainsbywetcontactprintingat differentmagnifications.Measuredandmodeledextinctioncross-sectionofthegoldnanoparticlelines.Natureofthepropagatingplasmonicmodes(red positiveandbluenegativecharge).(b)OpticalmicrographsandSEMimagesofdifferentassembledcolloidalparticlesonatwo-dimensionalhexagonal lattice[160].(c)Transmissionspectraofacolloidalmonolayerwithvariationoftheangleofincidence[161].ß(b)RoyalSocietyofChemistry,(c)Springer.
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RESEARCH:Review
ofopticalmodescouldbeexcited,namely(super-,sup-,andsub- sub-)radiantmodes(L1–L3)andonedipolarmode(T1)[58].Allof thesemodesdemonstratetheextentofspectralfine-tuning,which isprovidedbyarrangingsimplecolloidal buildingblocksonan additional hierarchical level. These effectsmay be used in the futuretoexcitespecificmodesasoutlinedinFig.5(b)forenhanced absorption.
Another way of template-free structuringand to controlthe transmissionisshowninFig.6(b)and6(c).Withasimpleinkjet printingapproachamonolayerofahexagonallyorderedpattern couldbefabricatedonvariouslengthscalesoftensofmmcompris- ingparticlesofvarioussizeinthevisibleregime[160].Thereflec- tanceof such longrangeordered colloidalmonolayerschanges accordinglyand the effectcanberecognizedwiththebare eye.
Thetransmissionpropertiesofsuchclose-packedcolloidalmono- layersundervariationoftheangleofincidenceisshowninFig.6(c).
Theminimuminthetransmissionspectrumisassociatedtolight diffractionparalleltothelatticesurface(Wood’sanomaly),which propagativemodesaretunablewiththeangleofincidence[161].
Inclosecontactwiththeactivelayerofasolarcell,suchreadily assembleddielectricnanospherescanbeusedtoconvertthefreely propagatingsunlightintoenergy.Figure7(a)showsatheoretical approachofwavelength-scaleddielectricspheres,whichenhance thesolarcell efficiency[77]via excitationofwhisperinggallery andwaveguidemodes.Thisstudyalsodemonstratedthepowerto
specificallytune the spectralresponse by colloidaldesign. Both particle diameter and particle spacing can beused to enhance absorption in the a-Si layer and thereby increase the current density. Furthermore, this improvement is retained at various angles of incidentlight, but features some dependence on the polarizationofthelight.Anexperimentaltwisttothisconcepthas been realizedby sphericalsiliconnanoshell arraysbyYao etal.
(Fig.7(b))[150].Inthisexperimentalstudytheshellmaterialitself constitutedtheactive(nc-Si)material.Theexcitationofwhisper- inggallerymodesalongthisshellstructureincreasedtheoptical pathlengthfrom50nm(planarreference)to1000nm.Incombi- nation with coupling between adjacent spheres and different particlesizes, anenhancedbroadbandabsorptioncouldbereal- ized.Whereasthesestructureshelptoenhancetheopticalabsorp- tion in particular for wavelengths beyond 500nm, the major challengeistoensuregoodelectricalcontacttoguaranteeahigh open-circuitvoltageandshort-circuitcurrentdensity.
Optical light-trappinggeometries and opticalresonancesdis- cussed here are not only restricted to colloidal particles. The optical principles can be expanded to inverted structures such asperiodicholestructures(part2).Thisrelationexpressedinthe Babinet’s principle,whichyields a correlation betweenthe dif- fractedlightofanopaquemaskandtheinvertedstructure.This principlewasoriginallyusedtosimplifytheanalysisofdiffraction problems[162].
FIGURE7
Usingwavelength-scaledielectricsphereassemblytoenhancesolarcellefficiency.(a)Schematicforresonantcouplingbyadielectricsphereinclosecontact toahigh-indexPVabsorberlayer(whisperinggallerymodes).Adjacent,calculatedopticalgenerationrateinthesiliconusingaflatfilmorthespherearray.
Cross-sectionofthesolarcelldeviceandthenormalizedintegratedelectricfieldandabsorptionintherespectivelayer[77].(b)Cross-sectionSEMimageof asphericalnanoshellmonolayerandcorrespondingabsorptionspectra(black:onelayer,red:twolayer,blue:threelayer,green:ITOasanti-reflecting coating)[150].ß(a)JohnWileyandSons,(b)NaturePublishingGroup.
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RESEARCH:Review
Template electrodes from colloidal assembly structures
Whereas the last section has discussed the use of assembled colloidsaslightmanagementstructuresinPV,acolloidalparticle layercanalsobeusedastemplatingstructureforfurthermodifi- cationofdevices.Thisconceptistypicallyknownasnanosphere lithography. This increases the parameter space compared to colloidalstructuresdiscussedbeforeandallowsadditionaldesign elementsinbothmaterialandnanostructuretotraplightinthin filmPVdevices.Figure8depictsthreesimplecategoriesoftem- platedelectrodedesignsalongwithexamplesfromliterature.We focusontwo-dimensionallyorderedparticlearraysbutnotethat otherassemblystructuresintroducedintheprevioussectionsmay also beusedfor templatingpurposes. Furthermore,colloidal li- thographyitselfrepresentsaveryactivefieldofresearch,which demonstrateda vast rangeofpossibleordered andnon-ordered nano-andmesostructures.Formoredetailsoncolloidallithogra- phy,we recommend the readerspecializedreviewsin thisfield [163,164].Here,werestrictourselvestoexamplesfromcolloidal lithographythatmadedirectcontributionstolightmanagement orphotovoltaics.
Thefabricationoftemplatedstructuresfortoporbottomelectro- descaneitherstartfromclose-packedcolloidalmonolayers(period- icitylequalsparticlediameter), ornon-close-packedarrays. The latterstructuresaretypicallyfabricatedusinganisotropicdryetch process,forinstancebyplasmaetching[170].Replicationofthis periodicstructurecanbeconductedwithvariousdegreesofcom- plexity.Moststraightforwardisthedepositionofadesiredmaterial intotheinterstitialspaceofaclose-packedmonolayer(Fig.8(a,c)).
Dependingontheamountandheightoftheaddedmaterialisolated triangularpyramidalstructures(Fig.8(f))[165]ornanobowlarrays (Fig.8(g))[166,171]areobtained.Onecanalsocapitalizeonthe chemicalcontrastbetweenthesurfaceandparticlearraymaterialto transfertheperiodicstructureintothesupportingsubstrateinstead ofbuildingitontop(Fig.8(d,h))[167].Byproperadjustmentofthe etchingparametersandgascompositiontheaspectratioandtaper- inggradientcanbetunedtogetnanopillarsaswellasnanocones fromthesamestartingmaterial.Non-close-packedcolloidalmono- layers (Fig. 8(b,e)) can be converted into nanomesh structures (Fig.8(k))[169].Mostcommonlythisnanomeshismetallicand canbeusedasatransparentconductingelectrodesimultaneously.
Onecanfurtherexploitthecontrastinmaterialschemistrybetween the nanomesh and support substrate. Similarly to the etching processemployedinFig.8(h),thenanomeshcanserveasaprotec- tivelayer.Therefore,onlythefreeholesaresubjectedtotheetching process,whichyieldsnanobowls(Fig.8(i))ornanowells,whichare embossedintothesupportstructure[141,168].Thestructuraldi- versityshowninFig.8isnotlimitedtoaspecifictypeofmaterial,but canberegardedasgenericmotif,whichinprinciplecanberealized bymetals,metaloxides,polymersandgraphiticentities.
Concomitantwiththestructuralrichnessofthetemplatedelec- trodes,thereisarangeofopticalproperties,whichcanleadtoan enhancedperformanceofthePVdevice.Theseincludelighttrap- pingbyscattering,excitationofphotonicwaveguides,orlocalfield enhancement by plasmonic resonances. Additionally, increased lightabsorption canbeachievedbythe antireflectiveproperties inparticularoftaperednanoconesandbyanangle-independent
FIGURE8
(a)Aclose-packedcolloidalmonolayerisimmobilizedonasubstrate.Thespherestypicallyconsistofpolystyrene(PS)orsilica.(b)Isotropicetchingreduces theparticlesizebutretainstheinitialperiodicity.(c–e)Arangeoftemplatedmotifsisaccessible:Isolatedpyramidsorcontinuousbowl-likefilmsremain afterdirectdepositionthroughtheorifices(f,g)[165,166].Exposingacolloidalmonolayertoreactiveionetchingyieldsnanopillars,nanodomes,and nanoconesdependingontherespectiveetchingrates(h)[167].Multiplelayersofphotoactiveandeletrodematerialcanbeconstructedontopofa templatedsubstrate(i)[168].Etchedcolloidalmonolayerscanbecoatedwithvariousmetalsresultinginmesh-likestructures,whichactasconducting electrodesthemselves(k)[169].ß(f )AIPPublishingLLC,(g)RoyalSocietyofChemistry,(h)IOPPUBLISHING,LTD,(i)NaturePublishingGroup.(k)AIP PublishingLLC.
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absorption.Continuous metallic nanostructures cannot onlybe employedasaconductingelectrodebutmayalsobeabletosupport propagatingsurfaceplasmons.Theseopticalcontributionsaresche- matically summarizedin Fig.9.Furthermore, when usingthese structuresaselectrodematerial,theinterfacialcontactareatothe photoactivelayerisincreasedcomparedtotheanalogflatstructure allowingforanincreasedchargecarrierextraction.
In the following we will discuss a range of nanostructured electrodes,whichcanbederivedbycolloidallithographystrate- giesasoutlinedinFig.8.Forreasonsofclarity,wereferto‘front’or
‘top’electrodewhentheelectrodeis(semi)transparentandfaces thedirectionofillumination.Wewillreferto‘back’or‘bottom’
electrodeforthenon-transparentelectrode.
Structuredelectrodesfromclose-packedcolloidalmonolayers Followingroute c fromFig. 8 allows one to fabricateinsulated structures, which typically possess a pyramidal shape with a triangularbase.Inthiscontextinsulatedmeansontheonehand aspatialseparationthatcanbeadjustedbythetemplatingcolloi- dal array (via l). On the other hand these structures are also electricallynon-continuousrequiringanadditionalelectriccon- tacttoprovidechargecarrierextraction.Sputteringorevaporation techniquescanbeusedtoobtaindefinedindividualstructuresofa certainheight.Lietal.sputteredTiO2ontoacolloidalmonolayer comprising500nm diameter polystyrene beads.These highre- fractiveindexstructuresof250nminheightontopofanAs/GaAs multiplequantumwellsolarcellfulfilledtwopurposes:broadband antireflectivepropertiesandexcitationofwaveguidemodesinto thequantumwelllayer.Theadvantageofthenanostructureovera flatlayerantireflectivecoatingisahigherlighttransmissionfor wavelengthsbelow600nmandtheadditionalexcitationofopti- calwaveguidemodesabove900nm,whichbothresultinahigher Jsc,ascanbeseeninFig.10(a) (bottomright).Furthermore,the nanostructured devices featured a strongly reduced sensitivity toward the angle of illumination, particularly for short wave- lengths (<600nm), which is important for non-tracking solar devices.However,thebenefitsgainedfromanincreasedabsorp- tion in the active layer are compromised by a concomitant
increaseinsurfacerecombination.Withoutsurfacerecombination effectsshort-circuitcurrentdensitiesofupto34.9mA/cm2could beexpectedinsteadof14.6mA/cm2,whichhasbeenmeasuredin thiswork(Fig.10(a)topright).
Inthefieldofnanospherelithographynoblemetalshavebeen widelyemployedassputteringorevaporationmaterialduetothe richopticalpropertiesoftheresultingplasmonicnanostructures.
Naturally,thesemotifshavealsobeeninvestigatedfortheirappli- cationinsolarcells[172,173].Incontrasttodielectricstructuresas describedabovetheevaporatedlayerthicknessestypicallyrangein thefewtensofnanometersforgold,silveroraluminum.Therefore themaingaininabsorptionefficiencyisnotderivedfromantire- flectiveproperties,butratherlocalplasmonicfieldenhancement FIGURE9
Schematicofopticalpropertiesofcolloidallytemplatedelectrodes.n denotesrefractiveindices:n1istypicallyglass(frontortopelectrode),n2
thephotoactivelayer,n3thecounterelectrodeoftenametal(backor bottomelectrode),andn4thetemplatedstructure.Whitearrowsindicate incidentlight,blackarrowsthelightafterpassingthetemplatedstructure.
(1)(back)scatteringoflight,(2)excitationofphotonicwaveguidemodesin theactivelayerbygratingcoupling,(3)antireflectivepropertiesofgraded interfacesandangle-independenceoflighttransmission(4)excitationof localizedsurfaceplasmons,(5)excitationofpropagatingsurfaceplasmons, (6)excitationofcavitymodes.
FIGURE10
(a)SputteredTiO2nanopyramidsontopofathinfilmquantumwellsolar cell.Acombinationofantireflectivecoatingandwaveguideexcitationin theactivelayerincreasestheJsccomparedtotheflatstructureor conventionalunstructuredARcoatings.Alongside,theEQEisincreased [165].(b)ThinfilmP3HT/PCBMsolarcellontopofananobowlAgbottom electrode.Localandsurfaceplasmonenhancementincreasesthe absorptionintheactivelayer[174].(c)Directetchingofnanoconesintoa glasssubstrate.Theabsorptioninamc-Sicellisincreased,whichcritically dependsontheperiodicityanddepthoftheetchedstructure[167].ß(a) AIPPublishingLLC,(b)OSA,(c)IOPPublishing.Reproducedbypermission ofIOPPublishing.Allrightsreserved’’.
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